The present invention is related to solid-state alternating current (AC) switches, and in particular to a solid-state AC switch that provides low electro-magnetic interference (EMI).
Solid-state AC switches are employed to control the supply of AC power to loads. A solid-state AC switch typically includes a pair of metal-oxide semiconductor field-effect transistors (MOSFETs) connected in series with one another. The source of a first transistor is connected to the AC input, with the drain/source of the first transistor connected to the drain/source of the second transistor. By selectively controlling the gate voltages of both the first transistor and the second transistor, the solid-state AC switch selectively supplies an AC input power to the respective AC load.
A pair of series connected semiconductor transistors is required to account for the positive and negative half-cycle of the AC input. A transistor acts like a diode when OFF (i.e., non-conducting), allowing uni-directional flow of power across the forward biased body diode of the transistor. Therefore, the first series-connected transistor prevents the flow of current during the positive half-cycle of the AC waveform and the second series-connected transistor prevents the flow of current during the negative half-cycle of the AC waveform.
Electromagnetic interference (EMI) is added to the AC source during turn-on of the solid-state AC switch if the current through the transistors is non-zero. Prior art embodiments therefore attempt to synchronize the turn-on of the first and second transistors with zero-crossings of the AC waveform. However, capacitance associated with each transistor (i.e., the Miller Effect) results in the storage and subsequent undesirable flow of current during turn-on of the solid-state AC switch that contributes to EMI generation.